Positron Emission Tomography (PET) is a procedure used for imaging and measuring physiologic processes within the human body. As part of the procedure, radioisotopes are injected into a patient to assist in diagnosing and assessing a disease. A radioisotope, such as Fluorine-18, may be produced through the use of a particle accelerator. In particular, the particle accelerator produces radioisotopes by accelerating a particle beam and bombarding a target material, housed in a target system, with the particle beam. Referring to FIGS. 1a and 1b, a general configuration for a particle accelerator 10 is shown. The particle accelerator 10 generates a particle beam 12 within movable concrete shields 14 and 16 and stationary concrete shields 18 and 20. The particle beam 12 then bombards target material 11 inside target enclosure 22 to produce a radioactive isotope. The resulting decay of the isotope as well as other interactions generate gamma rays and neutron particles 13 that are absorbed by the concrete shields 14, 16, 18,20 to protect any person located in areas outside of the concrete shields 14, 16, 18,20.
High density materials are used to shield a particle accelerator in order to reduce the energy from accelerated particles in the shortest distance possible. The effectiveness of a material in reducing energy or slowing down particles varies with different types of particles and their energy level. Concrete is typically used for shielding many types of radiation including gamma rays and neutron particles and is used extensively in the nuclear industry. In particular, the concrete shields 14, 16, 18,20 are made of concrete formed through use of a steel mold that is supported by reinforcement ribs. Additional shielding materials may also be mounted to the concrete shields 14, 16, 18,20 to improve the performance of the concrete shields. Reinforced steel and wood forms are typically used to mold the concrete used in stationary shield components such as walls, floors, and ceilings. In use, the shield components are typically covered with commercial materials such as sheet rock and paint to make their appearance more cosmetically acceptable. However, such commercial materials are labor intensive to use and lack durability.
In low energy cyclotrons and linear particle accelerators moveable concrete shields may be used in a self-shielded configuration. In these applications individual concrete pieces are also formed using steel molds. However, the molding process produces parts with poor surface finishes that require a great deal of manual labor to repair in order to produce a cosmetically acceptable surface. In addition, the resulting surface is brittle and is easily damaged when the steel mold is removed from the concrete and during the manufacturing, assembly, shipping and installation process. This requires constant repair of the damaged surfaces with the use of common filler materials, sheet rock mud and other repair materials in order to maintain the cosmetic appearance of the surfaces. However, such repair materials do not provide the same strength and shielding properties as concrete.
Concrete also has other structural limitations. Concrete has excellent compressive strength but is very weak in tension. These tensile strength limitations require that all concrete structural members be reinforced with steel reinforcement members 38 such as steel rebar in areas where the concrete structural members experience tensile loads. Use of steel to provide reinforcement is an undesirable feature in concrete radiation shields. The steel reinforcement members 38 become radioactive when acted upon by neutron particles 13 produced from the interaction of particle accelerator beam 12 on target material 11 inside target enclosure 22 whereas the surrounding concrete material does not. Thus, the reinforcement members 38 require additional shielding in the concrete shield. In addition, the steel reinforcement members 38, when radioactive, pose increased disposal cost to the customer when the particle accelerator is decommissioned at its end of life.
Further, the steel molds are subjected to frequent flexing, bending and movement during the fabrication of the concrete shields which quickly fatigues and damages the steel molds. As a result, the steel molds require frequent maintenance which increases costs. Moreover, the maintenance is labor intensive which further increases costs.
The manufacturing process for the concrete shields 14, 16, 18,20 takes place at the manufacturers' facility. The concrete shields 14, 16, 18,20 are then shipped to a customer location. The steel molds, on the other hand, are kept at the manufacturers' facility because their size and weight make them cost prohibitive to ship.
Therefore, new techniques are needed for casting shield components that reduce costs, are less labor intensive and result in shields that are cosmetically acceptable, provide enhanced structural characteristics and are durable and functional.